This invention is related to chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) apparatus, and to methods for maintaining such apparatus in good working order. In particular, it relates to gas and plasma distribution hardware (fluent material supply or flow-directing means with plasma) for vacuum coating of large-area substrates made of glass, other dielectrics, or other materials.
CVD and PECVD is a widely used method of applying precision coatings to substrates. Applications include semiconductor processing, optics, and specialty vessels and containers. In CVD, gaseous precursors containing particles of the desired solid material react and decompose at the substrate surface, leaving behind a solid coating. PECVD is a type of CVD that uses plasma excitation to accelerate precursor reaction. Among other advantages, the plasma excitation can enable deposition at reduced temperatures.
FIG. 1 is a schematic diagram of a basic CVD or PECVD chamber. The components needed for CVD are drawn in solid lines with plain reference numbers, and the extra components needed for PECVD are drawn with dotted lines and reference numbers in parentheses. Substrate 100 is held by holder 101 in a deposition chamber 102. Holder 101 may include control means for substrate temperature or surface charge to help particles 107 adhere to substrate 100. Chamber 102 may be operated at atmospheric pressure, low pressure, or high vacuum. Gaseous precursors enter through precursor inlet 103 and are evenly distributed in the vicinity of substrate 100 by passing through a precise array of holes 105 in precursor distributor 104. Precursor distributor 104 may be cooled. The precursors carry coating-material particles 107 to the surface of substrate 100, where they decompose out of the precursors and adhere to the substrate surface and to each other, forming the coating. Exhaust 106 removes waste components of the precursors, and at least some of the particles 107 that do not adhere to the substrate surface. Between deposition cycles, purging, passivating, or cleaning agents may be injected into chamber 102 through an auxiliary inlet 108.
In PECVD, the precursors are excited into a plasma by an electric field from a plasma excitation source (110). The electric field may be created on or in precursor distributor 104 via an electrical connection from excitation source (110) to precursor inlet 103. A ground connection (111) is provided for substrate 100. The plasma decomposes the precursors and deposits coating-material particles 107 on the surface of substrate 100, where they adhere.
As with similar coating methods, CVD and PECVD result in unwanted deposits on non-target surfaces, as well as those intended on the target substrate. Coating particles, reaction residues, and waste by-products are also deposited on other accessible surfaces inside the chamber: for example, on the inner chamber walls, the precursor distributor surfaces, the inlet and exhaust lines, and the substrate holder. These unwanted deposits can degrade the performance of chamber hardware. In particular, when unwanted deposits build up on the precursor distributor, some of its precision holes may become partially or completely blocked, degrading the efficiency and uniformity of the precursor distribution and the resulting coatings. If the precursor distributor is also a plasma source, deposit buildup may make the plasma more difficult to ignite, or render the electrodes more susceptible to arcing or overheating. Additionally, fragments or components of these unwanted deposits can detach from the chamber hardware during subsequent deposition cycles. If these fragments or components land on a substrate, they contaminate the composition or degrade the uniformity of the coating on that substrate. Therefore, if unwanted deposits cannot be avoided, they must regularly be removed from the chamber by cleaning or replacing the affected hardware.
Inventive work on removing deposits on chamber hardware falls into three main categories: (1) in situ non-invasive cleaning, where the chamber remains sealed during cleaning, (2) in situ invasive cleaning, where the chamber is opened and the hardware is cleaned in place, and (3) off-line cleaning, where the chamber is opened and the dirty hardware is removed, cleaned, and replaced, or removed and swapped out for clean replacements. For example, in U.S. Pat. No. 6,576,063, Toyoda et al. non-invasively clean the chamber and discharge tube using an etch-gas active species generated by a remote plasma source built permanently into the apparatus. In U.S. Pat. No. 6,110,556, Bang et al. machine specially designed throughways in a chamber lid to aim a cleaning gas asymmetrically toward the areas with the highest likelihood of unwanted deposits. In U.S. Pat. No. 4,657,616, Benzing et al. temporarily install a special cleaning manifold comprising a gas inlet, electrodes, and RF excitation electronics, unsealing the chamber but cleaning the hardware in situ and limiting the opportunity for contaminants to enter the chamber.
Some pieces of chamber hardware, and some types of deposits, do not respond well to convenient in situ methods. In addition, many in situ cleaning methods, apparatus, and agents are expensive. In such cases, the chamber must be opened and hardware must be removed for cleaning. In U.S. Pat. Nos. 6,374,841 and 6,234,219, Donohoe installs a removable chamber lining that can be replaced with a clean one, as do Frankel et al. in U.S. Pat. No. 6,444,037 and Chu et al. in U.S. Pat. No. 6,120,660. In U.S. Pat. No. 6,719,851, Kurita et al. introduce a chamber lid designed to be easily opened, closed, and rotated to facilitate wet cleaning of the chamber hardware. In U.S. Pat. No. 5,906,683, Chen et al. mount a baseplate with integral gas inlet and cooling-fluid inlet and outlet to reduce the number of vacuum-seal components in the lid that must be checked for intactness and carefully replaced during post-cleaning reassembly of the chamber.
Accumulation of unwanted deposits on precursor distributors can be especially problematic. Parts of these components are not easily accessed by in situ cleaning-agent flows. Gentle cleaning methods can take a long time to remove the deposits; and more vigorous cleaning methods can roughen or distort the precision edges of the precursor distribution holes. In U.S. Pat. No. 5,597,439, Salzman mounts a gas distribution ring with adjustable slots to a gap between an etch chamber's ceramic dome and its lower sidewall; the gap acts as a manifold channel when the chamber is sealed, and the distribution ring easily lifts out when the chamber is unsealed. In U.S. Pat. No. 5,997,649, Hillman stacks the precursor distributor “showerhead” (so called because the circular precursor distributors used for coating small circular substrates look like bathroom showerheads) with upper and lower insulators for easy removal and replacement. In U.S. Pat. Nos. 6,050,216 and 6,170,432, Szapucki et al. configure disk-shaped showerheads to easily mount and dismount with split collars made in two semicircular parts.
Modular approaches to semiconductor processing chambers are becoming popular, both for ease of invasive maintenance procedures and for configuration flexibility. In U.S. Pat. No. 5,948,704, Benjamin et al. use a universal housing with standardized mating mounts for a plasma source, substrate holder, and vacuum pump. In U.S. Pat. No. 6,890,386, DeDontney et al. build a precursor manifold out of modular pairs of gas injectors flanking a central exhaust outlet; the modules are easy to replace and the manifold can be easily built in a range of sizes. In U.S. Pat. No. 6,424,082, Hackett et al. use self-aligning contoured surfaces to enable quick replacement of consumable hardware in material-processing equipment, such as electrodes, swirl rings, nozzles, and shields. In U.S. Pat. No. 6,983,892, Noorbakhsh et al. bond the gas distributor to a removable electrode to form a removable module with space for an insert that prevents premature ignition.
Most of these prior-art advances in easily changeable precursor distributors are geared toward semiconductor processing, where the substrate is generally a circular wafer measuring 200 mm or less in diameter and the precursor distributor is configured to match. For substrates that are much larger, a single solid showerhead becomes too large and heavy for one technician to remove and replace easily. Besides, many large substrates that benefit from CVD and PECVD coating, such as solar panels, flat-panel displays, vehicle windshields, and architectural windows, are rectangular rather than circular. Even relatively small substrates, such as lenses and mirrors, are often coated in batches rather than one at a time, and a rectangular array of parts often allows better packing density than a circular array. These applications therefore require a very different plasma-delivery geometry and scale from those useful for wafer processing. Other rectangular substrates may require similar geometries.
FIG. 2 is a conceptual diagram of one type of precursor distributor designed for coating large rectangular substrates or rectangular arrays of substrates: the “tube-array showerhead.” (The diagram is not to scale and is not intended to represent any particular prior-art design). Instead of a single solid showerhead, the precursors flow into an array of removable tubes 201. Each individual tube may be long, but is narrow and lightweight enough for a single technician to maneuver. Tubes 201 are held along operating axes 2A, fed with precursors, and sealed by first manifold 202 and second manifold 203. The two manifolds 202 and 203 may or may not be symmetric. Precursors enter through at least one gas inlet 204 in one or both of the manifolds, and pass through to the processing chamber through multiple precision-drilled holes 205 in each of the tubes 201. Manifolds 202 and 203 may also include exhaust ports 206. For coating processes requiring sub-atmospheric pressure, manifolds 202 and 203 must seal ambient air out of the process chamber. The ends of tubes 201 must also be sealed to keep the incoming precursors channeled down the tubes and out through holes 205.
A tube-array showerhead can be used for PECVD if electrodes or other plasma excitation connections (212) are connected to the tubes. In that case, the tubes must somehow be insulated from the process chamber and from each other.
To preserve quality and uniformity in the coatings, the tubes in a tube-array showerhead must be cleaned whenever deposits build up on the tube surfaces and around the edges of the precision-drilled gas inlet holes. In situ cleaning is possible with some highly reactive cleaning agents, but these agents are very expensive and can greatly reduce the useful life of the showerhead, particularly if the holes are small. Removing the tubes and using a gentler cleaning solution is less expensive in terms of both cleaning agents and tube life.
To remove the tubes from a prior-art tube-array showerhead known to the inventor, one of the manifolds must be removed or partially disassembled to free one end of each tube. Next, the tubes are pulled free of the other manifold, which remains attached to the chamber. Clean or new tubes are then inserted into the manifold that remained attached to the chamber. Finally, the manifold that was removed or disassembled is replaced around the free ends of the clean tubes. In a mass-production environment, the tubes may need to be removed and cleaned weekly, or even more often.
FIG. 3 is a conceptual cross-section (through what would be section 2-2 in FIG. 2) of the assembled top portion of a tube-array showerhead according to a prior-art design. (In this design, the two manifolds are symmetric, so only one of them need be shown). The components needed for CVD are drawn in solid lines with plain reference numbers, and the extra components needed for PECVD are drawn with dotted lines and reference numbers in parentheses. Each end of tube 301 slides into a channel in manifold block 302, which is attached to chamber wall 300 by fasteners 303. Precursors from removable precursor inlet valve 307 enter the chamber through plenum inlet 308 to be distributed by channels in plenum 330 to each precursor inlet 304 in manifold block 302, and from there into tube 301, where they enter the chamber through precision holes 305. Waste products in the chamber are exhausted through exhaust port 306 in manifold block 302, back through one or more exhaust channels in plenum 330, through chamber wall 300, and into exhaust line 308. Tube seal 320 seals tube 301 to manifold block 302. Block seal 321 seals manifold block 302 to plenum 330.
The PECVD version of the prior-art tube-array showerhead of FIG. 3 requires additional components and functions. Plasma excitation source (310) sends its output to removable connector (314), which connects to bulkhead lead (313) through the chamber wall. When manifold block 302 is attached to plenum 330, bulkhead lead (313) makes electrical contact with electrode (312). Electrode (312), which has some spring-like qualities due to being hardened and bent, penetrates tube seal 320 to electrically contact conductive tube 301. This allows tube 301 to create an electric field that excites incoming precursors into a plasma. The need to confine the plasma electric fields requires a ground connection (311) on conductive manifold block 302, insulation (323) on bulkhead lead (313), and insulation (322) on electrode (312). In addition, tube seal 320 and block seal 321 need to be good insulators, or to be lined with good insulators, for PECVD operation.
When the tubes in this type of tube-array showerhead need cleaning or replacement, at least one of the manifold blocks must be unfastened and partially disassembled to disengage one end of each tube. Next, each tube must be extracted from the remaining manifold block. Each tube must be freed from its tube seals. In some PECVD designs, the electrodes must also be removed and replaced to access the tubes. All these procedures require the use of one or more particular tools. Performing these steps, and then reversing them to install clean tubes, can take 2 to 3 hours in a typical 48-tube chamber (not counting release and restoration of vacuum, or any purge or passivation steps). This frequent and extended down-time for the chamber is a disadvantage of the prior art because it adds to the cost of producing solar panels.
Every mechanical disturbance of a seal creates a risk of vacuum-chamber leakage. This is another disadvantage of the prior art in tube-array showerheads, because at least one block seal and all the tube seals are necessarily disturbed whenever the tubes are changed in the prior-art design, When surfaces of seals are exposed, particles may cling to them and they may be scratched or abraded. Any mechanical stress on a seal in an unintended direction, such as stretching, can enlarge existing defects. Even new replacement seals may be defective. Unfortunately, the only way to find a leak in the prior-art design is by fully reassembling the tube-array showerhead in the chamber, closing the chamber, and drawing down the pressure. If a leak is detected then, the tube-array showerhead must be disassembled again, and each seal examined carefully to identify the faulty one. This process can be difficult and time-consuming, and costly because the chamber remains inoperable.
Therefore, a need exists for a tube-array showerhead design that allows quicker removal and replacement of dirty or malfunctioning tubes, with a decreased risk of creating leaks in the process.